Nacreous pigments produce pearl-like, metallic, and iridescent effects. A widely used type comprises muscovite mica platelets coated with a metallic oxide, such as titanium dioxide. A relatively thin titanium dioxide coating produces a pearl-like or silvery luster. Mica platelets with thicker coatings produce color, even though the components are colorless, through the phenomenon of light interference. This type of coated platelets is known as an interference pigment. The color, called the reflection color, is seen most effectively by specular or mirror-like reflection, where the angle of reflection equals the angle of incidence. The reflection color is a function of optical thickness, i.e. the geometrical thickness times the refractive index, of the coating. Optical thickness of about 80 nm to about 140 nm produce reflections which may be called white, silvery or pearly while optical thickness of about 190 nm or more produces color reflections.
The pearlescent pigments which are most frequently encountered on a commercial basis are titanium dioxide-coated mica and iron oxide-coated mica pearlescent pigments. It is also well known that the metal oxide layer can be overcoated to achieve various desired effects. For instance, Linton, U.S. Pat. No. 3,087,828, describes mica coated with various oxides including those of titanium, iron, cobalt and chromium over which, if desired, a layer of calcined titanium dioxide can be positioned. Brand, U.S. Pat. No. 3,711,308, describes mica coated with a first layer which is a mixture of oxides of titanium and one or more metal oxides which can be, for instance, the oxides of iron, chromium and/or cobalt and a second layer of titanium dioxide. Franz, U.S. Pat. No. 4,744,838, describes coating mica with a layer of titanium dioxide and calcining to form two layers, an inner titanium dioxide layer and an outer pseudobrookite layer. A second layer of iron oxide is deposited and the pigment is recalcined.
Producing pearlescent interference pigments having black absorption colors which are high quality lustrous products has proven to be difficult. To obtain such products, use of chemical vapor deposition techniques is described, for instance, in European Patents 0579091 and 0571836. Techniques are known which use carbon black or titanium reduction as described, for instance, in U.S. Pat. Nos. 5,356,471 and 5,286,291. Unfortunately, these approaches have proven to be costly and/or impractical. Vapor deposition procedures are expensive and can have difficulty in producing the desired pigments in mass quantities required by the market. In addition, many pigments prepared in this manner need to be reheated at higher temperatures after initial production in order to achieve the desired result. Carbon black procedures are not very efficient since the majority of the pigment platelets being treated are not coated properly. The reduction techniques are very costly.
It is accordingly the object of the present invention to provide a new technique for producing lustrous, interference pigments having a black absorption color which overcomes difficulties encountered in the prior techniques and to provide high quality, lustrous, interference pigments having the black absorption color.